How do I know if a pokemon is cheated?

[Dogod]

I got a level 100 dialga off the GTS, and I think it's cheated. It was caught in a pokeball, not a special type, it has a frowning face next to its picture, and it was "apparently met at level 5" One more thing, it was met at Lake Verity on October 7, 2006. (the date is possible because it's Japanese)

Is it cheated? And if it is, what damage can it do to my game?

[Bullados]

Check its stats with the ones on Serebii.net. If it has greater stats than the maximum, given the nature, then it's most definately cheated. If cheated, it shouldn't really do anything to your game, really...

[venom000012]

the little face means that it used to have pokerus but once the pkrs goes away that little face appears letting you know that its gone

[Dogod]

The stats check up, is there another way I can know for sure?

[peepq]

its location and met level suggest that it was a starter pokemon....

i think its a fake

[Bullados]

If the stats check out, then it's fine. Even for Nintendo-sponsored events like 10th Anny, the only things that really matter are the stats and the moveset. If the stats and movesets are plausable for that Pokemon, then it's good.

Even if the Pokemon is cheated, it doesn't "infect" your game. It's just that one Pokemon that's a little off.

[karppa]

It must be a hacked Pokemon since it cannot be found or caught in Lake Verity, let alone at level 5. Also, it is possible to catch it in a Pokeball, since I caught my Dialga with a Pokeball. Best way to figure out if the Pokemon's legit or not, is to check the ball it's in (if it's in a Cherish Ball, and there have been no known events for it, then it's fake), check where it was caught, and check what level it was caught in. Here's a sample of where any legit Dialga/Palkia would be obtained:

It was caught in Spear Pillar.
The level it was caught in was level 47.
It was caught in any ball except the Cherish Ball.

Other than that, the only other way to find out is to try to calculate EVs, IVs and everything in between. In all reality, though, even if the catching info (as given in the sample above) seems legit, it doesn't necessarily mean that the Pokemon itself is legit, so be wary.